This invention relates to high-intensity discharge (HID) lamps and, more particularly to sintering aids for use with the electron emissive material used in the electrodes of such lamps.
High-pressure sodium-mercury vapor lamps have in the past utilized as electron emissive material a mixture of several oxide phases comprising thorium dioxide, barium thorate, dibarium calcium tungstate and barium oxide. This mixture of oxide phases was quite sensitive to the atmospheric contaminants with the result that even a brief exposure to air resulted in a relatively large pickup of water and carbon dioxide by the emission mixture. Such contaminants were rather difficult to remove. Silica (SiO.sub.2) was an effective sintering aid in connection with the foregoing emission material.
More recently, as disclosed in U.S. Pat. No. 3,708,710 dated Jan. 2, 1973 a high-intensity-discharge sodium-mercury vapor lamp which utilizes dibarium calcium tungstate as the electron emissive material, particularly effective in sodium vapor lamps employing polycrystalline alumina arc tube bodies, has been disclosed. Similarly, U.S. Pat. No. 4,052,634 dated Oct. 4, 1977 discloses an emission material for high-pressure mercury vapor and high pressure sodium vapor discharge lamps which consist mainly of one or more oxide compounds containing at least one of the rare earth metal oxides, alkaline earth metal oxide in a quantity of 0.66 to 0.40 mole per mole of rare earth oxide and at least one of the oxides of tungsten and molybdenum in a quantity of 0.25 to 0.40 mole per mole of alkaline earth metal oxide, the alkaline earth metal oxide consisting of at least 25 mole % of barium oxide. Additionally, the above referenced copending applications disclose new electron emissive materials for use in such lamps which comprise solid solutions of dibarium calcium tungstate (Ba.sub.2 CaWO.sub.6) and dibarium calcium molybdate (Ba.sub.2 CaMoO.sub. 6) and electron emissive materials consisting essentially of M.sub.3 M'.sub.2 M"O.sub.9, wherein: M is alkaline earth metal and at least principally comprises barium; M' is yttrium, a lanthanoid series metal, or any mixtures thereof; and M" is tungsten, molybdenum, or mixtures thereof.
These latter electron emissive materials after sintering usually have a consistency of a "soft powder" and small particles of the emission material can dust off during handling of the finished electrodes or even finished lamps. If this dusting does occur, the amount of electron emissive material retained on the electrodes will be reduced and may possibly shorten the life of the lamp. In addition, any dusting during lamp life can result in dark emission material particles depositing on the inside surface of the arc tubes, which deposited emission particles have a tendency to quickly spread and darken the arc tube and hence reduce the light output of the lamp. The use of silica (SiO.sub.2) as a sintering aid with these newer electron emissive materials has not been effective. Providing electron emissive material, after sintering, which has the consistency of a "soft brick" results in a significantly improved electrode structure and essentially eliminates any problem of dusting during handling of the finished electrodes and finished lamps.